1. Field of Invention
This invention is directed to iron-based powder compositions for use in powder metallurgy.
2. Description of Related Art
In general, an iron-based powder composition for powder metallurgy is produced by mixing an iron powder with an alloying powder such as copper powder, graphite powder or iron phosphide powder, and where needed, with a cutting improver powder, and a lubricant such as zinc stearate, aluminum stearate or lead stearate. Such a lubricant has heretofore been chosen by taking into account miscibility with metallic powder and freedom from elimination during sintering.
Recently, a growing demand has been made for the development of sintered materials with great strength. To cope with this trend, warm molding techniques have been proposed, which can form a molding having high density and great strength by molding a metallic powder with heating, as disclosed in Japanese Unexamined Patent Application Publication No. 2-156002 and Japanese Examined Patent Application Publication No. 7-103404 and U.S. Pat. Nos. 5,256,185 and 5,368,630. With regard to lubricants to be used in these molding techniques, importance is further placed on lubricity during heating in addition to the requirements of miscibility with metallic powder and freedom from elimination during sintering.
Namely, when being partly or wholly dissolved during warm molding, a lubricant needs to be uniformly dispersed in between the particles of a metallic powder. This brings about reduced friction resistance between the metallic particles and between the resultant compact and the corresponding die, consequently leading to improved compactibility.
However, the above-mentioned metallic powder composition has a first drawback that it causes undesirable segregation in its starting mixture containing an alloying powder and the like, and a second drawback that it suffers poor flowability during warming.
To alleviate the first drawback, i.e., to prevent the metallic powder composition from being segregated, a technique is known which employs a binding agent, as disclosed in Japanese Unexamined Patent Application Publication Nos. 56-136901 and 58-28321. However, when the binder is added in large amounts in preventing the metallic powder composition sufficiently from segregation, another problem arises that the powder composition becomes less flowable.
The present inventors have previously proposed to use as a binder a metallic soap or a composite melt composed of wax and oil, as disclosed in Japanese Unexamined Patent Application Publication Nos. 1-165701 and 2-47201. These techniques are capable of reducing segregation and dusting in a metallic powder composition to a markedly great extent, thus imparting improved flowability to the powder composition. But, the techniques are considered unsatisfactory in that the flowability of the powder composition becomes worse with time due to the means provided above for preventing the problem of segregation.
For that reason, the present inventors have further proposed that a high-melting composite melt composed of an oil and a metallic soap be used as a binding agent, as disclosed in Japanese Unexamined Patent Application Publication No. 2-57602. This technique has the advantage that such a composite melt does not vary significantly flowability with time, allowing the stock powder composition to be less susceptible to flowability variation even after a lapse of time. In such an instance, however, there is posed another problem that the powder composition becomes varied with respect to apparent density because an iron-based powder is mixed with a saturated fatty acid that has a high melting point and is solid at room temperature and with a metallic soap.
In an effort to solve this problem, the present inventors have proposed a technique in which an iron-based powder is coated on its surface with a fatty acid, followed by bonding additives to the coated surface of that powder with the aid of a composite melt composed of a fatty acid and a metallic soap, and by successive application of a metallic soap to the coated bonded surface of the iron-based powder. This technique is described in Japanese Unexamined Patent Application Publication No. 3-162502.
Segregation, dusting and other problems have been appreciably alleviated by virtue of the techniques disclosed in Japanese Unexamined Patent Application Publication Nos. 2-57602 and 3-162502, both of which are described above. However, these techniques provide no satisfactory solution to flowability. This is particularly true of flowability during heating in so-called warm compaction in which a powder composition after being heated up to about 150xc2x0 C. is filled in a die heated at a similar temperature and then is compacted.
Also, in the techniques of Japanese Unexamined Patent Application Publication No. 2-156002, Japanese Examined Patent Application Publication No. 7-103404 and U.S. Pat. Nos. 5,256,185 and 5,368,630 which, as cited above, are designed to improve compactibility in warm compaction, a metallic powder composition suffers from poor flowability during warming because a low-melting lubricant component forms wet crosslinking in between the metallic particles. Insufficient flowability makes compacting less productive, and moreover, causes irregular density in the compacts, which in some cases gives a sintered product having varied properties.
With regard to insufficient flowability of a metallic powder composition during warming that is described above as the second drawback, the present inventors have proposed processes for producing an iron-based powder composition, which are disclosed in Japanese Unexamined Patent Application Publications Nos. 9-104901 and 10-3 17001. Each such process permits an alloying powder to be free of segregation during warming and also permits a metallic powder composition to be more highly flowable during warming.
In the above processes, the alloying powder can be prevented from being segregated during warming, and the metallic powder composition can be improved in respect of flowability during warming, which effects are attained by the steps of coating at least one of an iron-based powder and an alloying powder with a surface-treating agent; mixing the iron-based powder and alloying powder with lubricants such as a fatty acid, a fatty acid amide and a metallic soap; after mixing, heating the resultant mixture at a temperature higher than the melting point of at least one of the lubricants, thereby melting at least one lubricant; cooling the mixture with stirring, thereby bonding an alloying powder to the surface of the iron-based powder; and, after further cooling, incorporating the cooled mixture with lubricants such as a fatty acid, a fatty acid amide and a metallic soap.
According to the techniques of Japanese Unexamined Patent Application Publication Nos. 9-104901 and 10-317001 both cited above, the flowability of the iron-based powder mixture in warm compaction is remarkably improved. From studies made by the present inventors, it has been presumed that such a desirable effect could be obtained by coating the surfaces of an iron-based powder and an alloying powder with a surface-treating agent composed of an organic component, thus decreasing the potential difference between the associated lubricants of low electric conductivity and the surface of the iron-based powder or alloying powder of high electric conductivity. This reduces the possibility of the iron-based powder or alloying powder sticking to the lubricants by contact electrification and enhances the possibility of the iron-based powder and alloying powder becoming wettable with the molten lubricants in a warm region. Unfavorably, however, such an iron-based powder composition is less flowable at relatively high temperatures. To ensure high flowability during warm compaction, the temperatures of the iron-based powder and the corresponding die should be strictly controlled. According to studies made by the present inventors, the insufficient flowability at relatively high temperatures mentioned above presumably results from too low a coating ratio of a surface-treating agent applied to the surfaces of an iron-based powder and an alloying powder. When being not coated with the surface-treating agent, the iron-based powder and alloying powder are less wettable with lubricants used. Immediately after the temperature rises above the melting point of one of the lubricants, the molten lubricant having stayed between the iron-based particles and the alloying particles forms wet crosslinking so that the powder composition becomes agglomerated and hence less flowable at relatively high temperatures.
In order to solve the foregoing problems of the conventional art, therefore, one object of this invention is to provide an iron-based powder mixture for use in powder metallurgy, which is superior in flowability and compressibility at from room temperature to a region of high warming temperatures, and is less dependent on temperature in respect of flowability and apparent density, as well as green density of constituent powders.
This invention also provides a process for producing such an iron-based powder composition.
Another object of this invention is to provide a method of forming an iron-based powder compact, which is capable of forming such an iron-based powder composition into a high-density iron-based powder compact.
The present inventors have conducted intensive research on factors that dominate the flowability properties of iron-based powder compositions, thus finding a that the flowability is largely influenced by the surface states of an iron-based powder and/or an alloying powder, particularly by the substances of layers formed on the powder surfaces and by the coating ratios of the layer. From continued research on the chemical structures of layers formed to coat the constituent powders, the present inventors have found that when being coated in a coating ratio of not smaller than about 80% with an organosiloxane layer, the powders become greatly wettable with a molten lubricant and hence give an iron-based powder composition with flowability improved at a markedly high level.
Furthermore, the present inventors have found that the temperature dependence of flowability in an iron-based powder composition is largely variable with the amounts of water which get absorbed on the surfaces of the constituent powders and vary with temperature increases.
It has also been found that the amounts of water eliminated with temperature increases and absorbed on the powder surfaces can be made less variable when the powder surfaces are coated with an organosiloxane layer in a coating ratio of greater than about 80%, such that the quantities of water molecules to be adsorbed to the powder surfaces at around room temperature are so controlled as to be constant. Still another finding is that when an organosiloxane layer is formed on the powder surfaces, the powders become highly wettable with lubricants, the iron-based particles become easily slidable at low temperatures (at around room temperature) so that they are speedily rearranged to improve compression density at low temperatures and to reduce temperature dependence during compacting.
This invention has been made with further consideration given to the above findings.
More specifically, according to a first aspect of this invention, there is provided an iron-based powder composition for use in powder metallurgy comprising an iron-based powder, a lubricant melted and fixed to the iron-based powder, an alloying powder bonded to the iron-based powder with the aid of the lubricant, and a free lubricant powder. At least one member selected from the group consisting of the iron-based powder, lubricant, alloying powder and free lubricant powder is coated on the surface thereof with an organosiloxane in a coating ratio of greater than about 80%.
Preferably, the organosiloxane has phenyl groups as a functional group. The lubricant is one member selected from the group consisting of a composite melt composed of a calcium soap and a lithium soap, and a composite melt composed of a calcium soap and an amide lubricant. The free lubricant powder is one member selected from the group consisting of a mixed powder composed of an amid lubricant and a methyl polymethacrylate powder, and a lithium soap powder.
Also, the amide lubricant is preferably represented by the following formula:
CzH2zxe2x88x921CONH(CH2)2NH(CO(CH2)8CONH(CH2)2NH)xCOCyH2y+1xe2x80x83xe2x80x83(1)
where the subscript x denotes an integer of from 1 to 5, the subscript y denotes an integer of 17 or 18, and the subscript z denotes an integer of 17 or 18.
Also, the methyl polymethacrylate powder is preferably an agglomerate in which spherical particles are preferably with an average diameter in the range of from about 0.03 to about 5 xcexcm. The average diameter of the agglomerate is preferably in the range of from about 5 to about 50 xcexcm.
The free lubricant powder is present preferably in the range of from about 25 to about 80% by mass relative to the total amount of the lubricants.
According to a second aspect of this invention, there is provided a process for producing an iron-based powder composition for use in powder metallurgy comprising: coating at least one of an iron-based powder and an alloying powder with an organoalkoxysilane that has been mixed in advance with water; primarily mixing the iron-based powder and the alloying powder by the addition of one or more lubricants; heating the primary mixture with stirring at a temperature higher than the melting point of at least one of the lubricants, thereby melting at least one lubricant; cooling the mixture, wherein at least one lubricant has been melted, with stirring, thereby bonding the alloying powder to the iron-based powder with the aid of at least one lubricant, which has been melted and fixed to the surface of the iron-based powder; and subsequently performing secondary mixing by the addition of one or more lubricants.
In the second aspect, preferably when two or more lubricants are used in the primary mixing, the lubricants have respective different melting points. Also, one or more lubricants used in the primary mixing is preferably selected from a mixture composed of a calcium soap and a lithium soap, and a mixture composed of a calcium soap and an amide lubricant, whereas one or more lubricants used in the secondary mixing are selected from a mixed powder composed of an amide lubricant and a methyl polymethacrylate powder, and a lithium soap powder.
In the second aspect, preferably the amide lubricant is represented by the following formula:
CzH2z+1CONH(CH2)2NH(CO(CH2)8CONH(CH2)2NH)xCOCyH2y+1xe2x80x83xe2x80x83(1)
where the subscript x denotes an integer of from 1 to 5, the subscript y denotes an integer of 17 or 18, and the subscript z denotes an integer of 17 or 18. Also, the methyl polymethacrylate powder is preferably an agglomerate in which spherical particles have been agglomerated preferably with an average diameter in the range of from about 0.03 to about 5 xcexcm. The average diameter of the agglomerate is preferably in the range of from about 5 to about 50 xcexcm.
In the second aspect, preferably the amounts of one or more lubricants used in the secondary mixing are preferably in the range of from about 25 to about 80% by mass relative to the total amount of the lubricants used in the primary and secondary mixing.
In the second aspect, preferably the lowest-melting lubricant of the one or more lubricants used in the primary mixing has a lower melting point than the lowest- melting lubricants of the one or more lubricants used in the secondary mixing, and the heating temperature during the primary mixing is set to be between the melting points of the two lowest-melting lubricants.
According to a third aspect of the invention, there is provided a process for producing an iron-based powder composition for use in powder metallurgy comprising: primarily mixing an iron-based powder and an alloying powder by the addition of one or more lubricants; heating the primary mixture with stirring at a temperature higher than the melting point of at least one of the lubricants, thereby melting the one lubricant; cooling the mixture, wherein at least one lubricant has been melted, with stirring, mixing an organoalkoxysilane that has been mixed in advance with water, in the course of cooling and in a temperature region of from about 100 to about 140xc2x0 C., and bonding the alloying powder to the iron-based powder by the use of the at least one lubricant, which has been melted and fixed to the surface of the iron-based powder; and subsequently performing secondary mixing by the addition of one or more lubricants.
In the third aspect, preferably when two or more lubricants are used in the primary mixing, the lubricants have respective different melting points. Also, preferably one or more lubricants used in the primary mixing are selected from a mixture composed of a calcium soap and a lithium soap, and a mixture composed of a calcium soap and an amide lubricant, whereas one or more lubricants used in the secondary mixing are selected from a mixed powder composed of an amide lubricant and a methyl polymethacrylate powder, and a lithium soap powder.
In the third aspect, preferably the amide lubricant is represented by the following formula:
CzH2z+1CONH(CH2)2NH(CO(CH2)8CONH(CH2)2NH)xCOCyH2y+1xe2x80x83xe2x80x83(1)
where the subscript x denotes an integer of from 1 to 5, the subscript y denotes an integer of 17 or 18, and the subscript z denotes an integer of 17 or 18. Preferably, the methyl polymethacrylate powder is an agglomerate in which spherical particles have been agglomerated preferably with an average diameter in the range of from about 0.03 to about 5 xcexcm. The average diameter of the agglomerate is preferably in the range of from about 5 to about 50 xcexcm.
In the third aspect, preferably the amounts of one or more lubricants used in the secondary mixing are in the range of from about 25 to about 80% by mass relative to the total amount of the lubricants used in the primary and secondary mixing. Also, preferably the lowest-melting lubricant of one or more lubricants used in the primary mixing has a lower melting point than the lowest-melting lubricants of one or more lubricants used in the secondary mixing, and the heating temperature during the primary mixing is set to be between the melting points of the two lowest-melting lubricants.
According to a fourth aspect of the invention, there is provided a method of forming an iron-based powder composition into a high-density, iron-based powder compact comprising compacting an iron-based powder composition according to the first aspect at a temperature within the range of higher than the lowest melting point of, but lower than the highest melting point of, lubricants contained in the iron-based powder mixture.